Format rack-attack logging as JSON

[zachmargolis]

Why:
Was previously being logged as a Ruby hash, which ELK doesn't parse

Tested by tailing log/development.log

before:

{:discriminator=>nil, :event=>"throttle", :type=>"allow from localhost", :user_ip=>"127.0.0.1"}

after:

{"discriminator":null,"event":"throttle","type":"allow from localhost","user_ip":"127.0.0.1"}

[monfresh]

lgtm % minor Code Climate issue

[monfresh]

Also, I believe you might need to update the tests. They expect a Hash, not JSON.

[zachmargolis]

@monfresh thanks! updated the tests